1. Field of the Invention
The present invention relates to an electric motor assist type power steering apparatus. More specifically, the present invention relates to an electric-motor-controlled power steering apparatus, which reduces the steering effort of a driver by applying power (steering torque) generated by an electric motor to a steering system.
2. Prior Art
An electric motor assist type power steering apparatus (hereinafter defined as xe2x80x9cpower steering apparatusxe2x80x9d) supplies an assistant steering force to a steering system in order to assist a driver in turning a steering wheel. Vehicles equipped with power steering apparatus are in widespread use. In such vehicles, since smooth turning of the steering wheel is enabled by the power steering apparatus, the driver can turn the steering wheel with ease.
As an example of these kinds of power steering apparatus, a pinion assist type electric power steering apparatus, in which a worm shaft and a worm wheel are used in order to transmit an assistant steering force generated by an electric motor to a steering shaft, is known. The power steering apparatus, disclosed in the Japanese patent unexamined patent publication H11-43062, is one example thereof.
In this power steering apparatus, as shown in FIG. 8, an electric motor 51, which is used to provide an assistant steering force to a steering shaft, is provided. When the steering wheel is rotated in order to turn the steering shaft, the electric motor 51 supplies the assistant steering force to help rotate the steering shaft. The electric motor 51 is connected to a worm shaft 52. A worm gear 52A of the worm shaft 52 is engaged with a worm wheel 54, which is coaxially arranged with respect to a pinion shaft 53.
Ends 52B and 52C of the worm shaft 52 are supported by ball bearings 55 and 56. The assistant steering force generated by the electric motor 51 is transferred to the pinion shaft 53 through the worm gear 52A and the worm wheel 54, which are engaged with each other. Thereby, the effort that must be exerted by a driver to turn the steering wheel is alleviated by the assistant steering force.
In the power steering apparatus 50 disclosed in the above described. Japanese unexamined patent application, however, ends 52B and 52C of the worm shaft 52 are only supported by ball bearings 55 and 56, with the occurrence of backlash.
Therein, since the flexural rigidity (also referred to as flexural resistance, the capacity of an object to resist flexing when stress is applied to thereto) of the worm shaft 52 is relatively low, the worm shaft 52 tends to be easily deformed by the load applied to the worm shaft 52. Therefore, when the steering wheel is turned back in the other way, since the steering torque sensed by a torque sensor (not shown) decreases, the electric motor 51 is controlled so as to provide a decreased assistant steering force.
It is desirable to enable the steering wheel to be turned back smoothly, by rotating the electric motor 51 through the worm wheel 54 and worm shaft 52. But, if the worm shaft 52 has flexed, since the assistant steering force has decreased, the bending moment for affecting the worm shaft 52 decreases. When a restoring force is applied to the worm shaft 52, restorative deformation of the worm shaft 52 occurs.
Since the restorative deformation disturbs the rotation of the worm gear 52A, the assistant steering force from the electric motor 51 is not transmitted to the worm wheel 54. Thus, the proper relationships between the torque sensor, the control unit, and the electric motor 51 are impaired. These relationships are further impaired due to the moment of the inertia of the electric motor 51. Thus, the operational ability of the steering wheel is becomes worse.
In the electric power steering apparatus 50, moreover, an excessive gap or clearance may develop between the worm gear 52A and the worm wheel 54 as the worm shaft 52 flexes. When such clearance arises, the backlash between the worm gear 52A and the worm wheel 54 can become large. Thus, the assistant steering force generated by the electric motor 51 cannot be transferred completely to the steering shaft.
These disadvantages caused by the distortion of the worm shaft 52 are especially pronounced when the assistant steering force generated by the electric motor 51 is large.
The present invention relates to an electric power steering apparatus, that transmits a steering force to be added to the steering wheel, in compliance with the steering force exerted by the driver.
This electric power steering apparatus includes an electric motor for generating the assistant steering force, a worm shaft engaged with the electric motor so as to be rotated thereby, a worm gear disposed on the worm shaft and engaged therewith so as to rotate therewith, a worm wheel engaged with the worm gear on the worm shaft so as to be rotated by the worm gear, and a pinion shaft engaged with the worm wheel so as to rotate therewith. The present invention also includes a rack shaft engaged with at least one steered wheel so as to displace the steered wheel in an axial direction of the rack shaft, and first and second supporting units supporting the first and second ends of the worm shaft. The present invention further includes a distortion prevention unit engaged with the worm shaft at the middle thereof.
In the electric power steering apparatus according to the present invention, both ends of the worm shaft are supported by the supporting units without play. Thus, the worm shaft can be supported with high flexural rigidity.
Accordingly, since distortion of the worm shaft is avoided, the steering force generated by the electric motor may be reliably transferred to the pinion shaft. Difficulties with the operational ability of the steering wheel thus may be avoided.
In the electric power steering apparatus, preferably, each of the supporting units is composed of ball bearings for supporting their respective ends of the worm shaft.
According to this electric power steering apparatus, both ends of the worm shaft are supported without play using ball bearings. Thus, the worm shaft can be rotated while being well supported. Because ball bearings are general-purpose parts, this may be accomplished without using specialized parts. Consequently, a cost reduction may be attained.
Alternatively, in the electric power steering apparatus, preferably, the supporting units include ball bearings which support the first end of the worm shaft, and a needle bearing which supports the second end of the worm shaft.
According to this electric power steering apparatus, the first end of the worm shaft that is joined with the electric motor is supported by ball bearings. Since the first end of the worm shaft is joined with the electric motor, flexural rigidity in the first end of the worm shaft is comparatively high. However, if left unsupported, the flexural rigidity of the other end becomes relatively low.
In the present electric power steering apparatus, therefore, the second end of the worm shaft is supported by a needle bearing, without play. When the worm shaft is supported by the needle bearing without play, the flexural rigidity along the worm gear as a whole may be higher. Difficulties with the operational ability of the steering wheel thus may be avoided.
Furthermore, the present invention relates to an electric power steering apparatus including an electric motor generating an assistant steering force to be added to the steering system, a worm shaft engaged with the electric motor so as to be rotated thereby, a worm gear disposed on the worm shaft and engaged with the worm shaft so as to rotate therewith, a worm wheel engaged with the worm gear on the worm shaft so as to be rotated by the worm gear, pinion shaft engaged with the worm wheel so as to rotate therewith, and a rack shaft engaged with at least one steered wheel so as to displace the steered wheel along the axial direction of the rack shaft. First and second supporting units support the first and second ends of the worm shaft while allowing rotation of the worm shaft, and a distortion prevention unit is engaged with the middle of the worm shaft while allowing rotation of the worm shaft.
The electric motor, worm wheel, and supporting units may be referred to collectively as a torque transmission unit.
According to this electric power steering apparatus, the middle in the longitudinal direction of the worm shaft is supported by the distortion prevention unit while allowing rotation of the worm shaft. Thus, the worm shaft can be made to rotate, and distortion of the worm shaft can be prevented. Thus, difficulties with the operational ability of the steering wheel thus may be avoided, and the steering force generated by the electric motor can be transmitted reliably to the pinion shaft. With the use of a distortion prevention unit, the supporting unit at the first end of the worm shaft may be omitted, and the assembling efficiency of the apparatus as a whole is improved.
In the electric power steering apparatus, it is preferable that the distortion prevention unit includes an engagement assistor, which pushes the worm shaft into engagement against the worm wheel.
According to the present invention, since the engagement assistor pushes the worm shaft into engagement with the worm wheel, distortion of the worm shaft is prevented, and the worm shaft is pressed to the worm wheel with sufficient force so that the clearance between the worm shaft and the worm wheel is maintained within the predetermined range. The occurrence of the unpleasant backlash between the worm gear and the worm wheel can be prevented, and the rotation force of the worm shaft can be reliably transmitted to the worm wheel.
In the electric power steering apparatus, it is preferable that the engagement assistor includes a first roller, a second roller adjacent to the first roller, and a spring that pushes the first and second rollers against the worm shaft.
In this electric power steering apparatus, the worm shaft is pushed against the worm wheel with the worm shaft central to the first roller and the second roller. Thus, displacement of the worm shaft in an up-and-down direction is restricted, and the engagement assistor engages the worm shaft is with the worm wheel. Since rollers are used, friction is low, and the rotation of the worm shaft is not prevented.